The present invention relates to an optical isolator independent of polarization and to an optical fiber amplifier using the optical isolator.
An optical fiber amplifier is for amplifying signal light by utilizing stimulated emission caused by a population inversion in a laser medium in an optical fiber. A commonly used laser medium is an erbium-doped optical fiber (EDF) produced by adding erbium (Er), which is one of rare earth elements, to the core of a quartz-type optical fiber. The amplification of signal light is achieved by injecting intense excitation light as well as the signal light into the EDF. Excitation systems whereby excitation light is injected into the EDF include: a forward excitation system whereby excitation light is made incident from the same direction as the propagation of signal light; a backward excitation system whereby excitation light is made incident from the direction opposite to the propagation of signal light, and a bidirectional excitation system whereby excitation light is made incident from both directions. The forward excitation system is effective in reducing the noise figure (NF), while the backward excitation system is effective in obtaining a high-power output. The bidirectional excitation system has the advantage of the forward excitation system and the advantage of the backward excitation system in combination. These excitation systems are selectively used depending on characteristics required, such as amplification output, input optical power, gain, and NF.
The basic optical fiber amplifier is composed of an EDF, an excitation light source, an optical multiplexer, and an optical isolator independent of polarization (hereinafter referred to as an optical isolator). The optical isolator, which is for preventing the oscillation of the optical fiber amplifier, is generally connected to each end of the EDF.
Conventionally, there has been proposed an optical fiber amplifier in which a single optical isolator is connected to both ends of an EDF so as to perform a function comparable to the function of the two optical isolators (Japanese Laid-Open Patent No. 4-190333 or 6-118235).
However, the conventional optical isolator is disadvantageous in that a larger mounting area is required for intricate installation of optical fibers, since an input optical fiber and an output optical fiber are connected on both sides of the main body of the optical isolator.
Moreover, since the input optical fiber and output optical Fiber are connected on both sides of the main body of the optical isolator, it is necessary to adjust the optical axes of the input optical fiber and output optical fiber individually, resulting in a larger number of points at which the optical axes should be adjusted.
Furthermore, if the function of monitoring a part of signal Light and an optical multiplexing function are to be imparted the conventional optical isolator, the structure thereof becomes complicated as well as the parts thereof increase in size.